1. Field of the Invention
The invention relates to optical disks, and more particularly to disk type identification.
2. Description of the Related Art
When an optical disk is inserted into an optical disk drive, the optical disk drive first has to identify the disk type, after which data of the optical disk can be retrieved accordingly. Different disk types have different distances between polycarbonate layers and reflection layers thereof. A reflection layer forms a reflection surface reflecting a radio frequency signal emitted by a pickup head of an optical disk drive, thus data stored as pits and lands on the reflection surface is read by the pickup head. A polycarbonate layer forms a transparent layer laying over the reflection layer for protection. For example, the distances of CDs, DVDs, and Blu-ray disks are respectively 1.2 mm, 0.6 mm, and 0.1 mm. Consequently, disk type can be identified according to the distance between a polycarbonate layer and a reflection layer of the optical disk.
FIG. 1 is a schematic diagram showing motion of a pickup head during disk type identification. The pickup head projects a radio frequency signal on an optical disk. A surface 102 is the polycarbonate layer surface of the optical disk, and the surfaces 104, 106, and 108 are respectively reflection layer surfaces corresponding to the optical disk of BD, DVD, and CD categories. Thus, the radio frequency signal is first partially reflected by the polycarbonate layer surface 102 to form a first peak in the reflection signal, and then fully reflected by one of the reflection layer surfaces 104, 106, or 108 based on the disk type to form a second peak in the reflection signal.
When the disk identification begins, the pickup head starts to move in a direction perpendicular to the optical disk surface with an initial position 110, progressively approaching the optical disk. When the pickup head moves to the position 112, the pickup head receives the first peak of the reflection signal induced by the polycarbonate layer surface 102. When the pickup head further moves to the position 116, if the optical disk is a DVD, the pickup head receives the second peak of the reflection signal induced by the reflection layer surface 106. If the optical disk is a CD, when the pickup head further moves to the position 118, the pickup head receives the second peak of the reflection signal induced by the reflection layer surface 108. Increased distance between the polycarbonate layer surface 102 and the reflection surface requires more time to move the pickup head from the position 102 to the position 106 or 108. Thus, if the optical disk drive records the period between the first peak occurrence and the second peak occurrence, the disk type can be determined, due to the length of the period reflecting the distance between the polycarbonate layer surface and the reflection layer surface.
The conventional method; however, presents certain problems. When the pickup head moves to the position 112 where the first peak induced by the polycarbonate layer surface 102 occurs, the velocity of the pickup head does not hold on constant due to various reasons such as a tilting turntable loading the optical disk. FIG. 2A shows a tilted turntable with the pickup head and the optical disk separated by a short distance. The pickup head is accelerated from the initial position 202 and a first peak occurs in the reflection signal when the pickup head moves to the position 204. Because the accelerating distance between the initial position 202 and the position 204 is small, the velocity V1 of the pickup head at the position 204 is slow. FIG. 2B shows a tilting turntable with the pickup head and the optical disk separated by a long distance. Because the accelerating distance between the initial position 252 and the position 254 is long, the velocity V2 of the pickup head at the position 254 is high. Thus, although the distances between the layers 212 and 214 and the layers 262 and 264 are both d, the periods between a first peak occurrence and a second peak occurrence are respectively (d/V1) and (d/V2), which are not the same.
Other factors may also cause velocity difference of the pickup head. For example, the motion of the pickup head is driven by a focusing actuator signal, if the signal gain or sensitivity of the optical disc drive amplifying the focusing actuator signal is not constant over all optical disk drives due to component deviation, velocities of pickup heads of each drive also differ. Additionally, the distance between the initial position of the pickup head and the turntable varies over the optical disk drives, also causing velocity difference. Furthermore, a various of factors may also effect the accuracy of sensitivity of the optical disc drive, such as the deficiencies of pick-up head, different components, different product manufactures, and etc. . . . Thus, if the same fix threshold is used to compare with the period between a first peak occurrence and a second peak occurrence of the reflection signal to identify disk type, misidentification may occur, causing errors in data reading. A method for identifying disk types of optical disks is therefore required to solve the problem.